Q: What do you get if you cross a mosquito with a mountain climber?
A: You can’t cross a vector with a scalar.

Q: How many topologists does it take to change a lightbulb?
A: Just one, but what will you do with the doughnut?

Q: Why did the chicken cross the Möbius strip?
A: To get to the other–er….

If you understand what Bourbaki is all about then this one is lovely:

Q: How many Bourbakists does it take to replace a lightbulb?
A: Changing a lightbulb is a special case of a more general theorem concerning the maintenance and repair of an electrical system. To establish upper and lower bounds for the number of personnel required, we must determine whether the sufficient conditions of Lemma 2.1 (Availability of personnel) and those of Corollary (2.3.55 Motivation of personnel) apply. If and only if these conditions are met, we derive the result by an application of the theorems in Section 3.1123. The resulting upper bound is, of course, a result in an abstract measure space, in the weak-* topology.